1. Field of the Invention
The present invention relates to thermal treatment of glass sheets to provide a break pattern that represents an improvement over the typical break patterns that are produced when glass sheets are either tempered or heat strengthened by conventional techniques of heating to above the strain point of the glass and a temperature sufficient for tempering followed by cooling at a controlled rate of cooling or are heat treated by heating the glass sheets to a temperature sufficient for tempering followed by natural cooling.
It is well known that when glass sheets are subjected to a thermal treatment consisting of heating followed by rapid cooling, that the glass sheets develop a temper. The degree of the temper depends upon the elevated temperature to which the glass sheets are heated and the rate of cooling from the elevated temperature to below the strain point of the glass. Tempered glass sheets develop a compression stress zone in their edge and surface portions that surround an interior portion stressed in tension. Since glass is extremely strong in compression and extremely weak in tension, as long as any mechanical or other force applied to the tempered glass is insufficient to overcome the compressive stress at its surface or edge portion, tempered glass resists breakage. However, when glass sheets that are tempered do fracture, they form relatively small particles that separate readily from a frame in which the window is installed in a building. Fragments dropping onto a pavement below the window are dangerous to passersby.
When fabricating windows for skyscrapers, considerable breakage has occurred. Glass sheets that are heat strengthened in an attempt to provide at least a partial temper, when broken due to stresses resulting from mechanical or heat forces applied locally to portions of a heat strengthened window, develop a break pattern. Under certain conditions, the break pattern provides a line of breakage that is almost continuous to from one or more discrete areas inward from the frame that supports the window. Large pieces of glass within said frame sometimes separate from the window and fall onto the sidewalk below.
Uncontrolled tempering of glass sometimes causes glass warpage which results in mechanical stress during and after installation. Inducing a partial temper by controlling heating and/or cooling has been used to develop windows that are warped a minimum amount so that they can be installed without excessive stresses within a window frame. However, under some circumstances, certain heat strengthened glass sheets have fractured in the past, and the resulting break pattern provided at least one large piece that fell out of the remainder of the window installed within an installation frame.
Attempts to develop a better breakage pattern that does not result in a breakage line enclosing a discrete area spaced inward from the installation frame is generally associated with providing heat strengthened glass sheets that have a lower stress pattern than tempered glass. A suggested method of producing a lower stress pattern involves a slower rate of forced cooling, even to the point of letting the glass cool naturally. It has been found that under some conditions such naturally cooled glass also develops a break pattern that includes a break line surrounding a discrete area of rather large dimensions that tends to break away from the remainder of the glass sheet. Reducing the rate of applying air to the opposite surfaces of the heated glass sheets reduces the stress in the glass but, because the stress reduction is insufficent, fails to avoid the break pattern that includes discrete areas likely to fall on people walking in the vicinity of skyscrapers.
Prior to the present invention, a need existed for a window which would be retained within an installation frame when it developed cracks and a method for making a window having a break pattern that insured the likelihood of such an event that would avoid the dropping of large pieces from windows onto pedestrians below a building in which the window is installed. Generally, glass sheets were rendered less susceptible to thermal breakage and to edge damage during handling and installation by imparting a high edge compression stress to the glass. However, glass sheets thermally treated to have a high edge stress also had a steep surface compression gradient throughout the extent of the glass sheets. The prior art did not fully appreciate the correlation of a steep surface compression stress pattern with a poor break pattern.
2. Description of the Prior Art
U.S. Pat. No. 2,093,040 to Eckert teaches a two step method of tempering glass sheets in which the glass sheets are initially chilled as rapidly as possible to a temperature which lies at or near or somewhat below the annealing temperature of the glass, i.e., that temperature below which temporary stresses are mainly developed. According to this patent, further cooling is accomplished at a slower rate, but one that is still more rapid than a natural cooling rate in an open air environment.
U.S. Pat. No. 2,188,401 to Crowley discloses apparatus for tempering glass sheets in which a plurality of rotatable shutters are interposed between upper and lower sets of nozzles to insure that the entire length of a glass sheet is cooled from the same instant at the beginning of a cooling step as all other portions of the sheet so as to minimize the danger of warpage or breakage of the sheet during its fabrication.
U.S. Pat. No. 3,251,670 to Acloque interposes a disc or a donut-shaped member between tempering nozzles and a portion of the glass sheet to be provided with less temper than the remainder of the sheet in a technique for differentially tempering glass sheets. Other patents showing deflectors or angle bars interposed between a source of pressurized air and the opposite major surfaces of a glass sheet to be differentially tempered include U.S. Pat. Nos. 3,363,936 and 3,396,001 to Baker and U.S. Pat. No. 3,364,006 to Newell et al.
Furthermore, U.S. Pat. No. 3,304,166 to Bolland discloses the use of screens for reducing the rate of flow from high pressure air blasting members against a localized portion of a glass sheet to be tempered to a lower stress than the remainder of the sheet during differential tempering.
U.S. Pat. No. 3,847,580 to Misson discloses a two step cooling method for tempering glass sheets while supported on a gaseous hearth. During the first step, the glass is supported and chilled rapidly until both its center plane temperature and its surface temperature is reduced below the strain point of the glass. During the second step, the glass is supported and cooled by relatively lower volumetric flows of cooling gas per unit of surface support area to maintain the temper initially imparted during the first step. The total power consumption for this two stage tempering process is less than that required for conventional tempering in which the high rate of cooling is maintained throughout cooling.
Other patents reported in a novelty search report include U.S. Pat. No. 2,959,836 to Hanley which was cited for showing a plurality of valves or dampers provided to control the air volume exhausted from a cooling section of a glass heat treating apparatus, and U.S. Pat. No. 3,986,856 to Fournier et al. recited for disclosing a glass sheet treatment apparatus in which the application of air is interrupted when there is no workpiece facing air delivery apparatus.